This invention relates to a copolyester of polyethylene terephthalate and a process for preparing the copolyester. The copolyester is useful in forming films and clear molded articles. More particularly, this invention relates to a copolyester of polyethylene terephthalate and sulfonyl diphenol having an increased crystallization time which provides the user of such copolyesters with more processing time to manufacture thicker films or molded articles, thereby avoiding premature crystallization and its attendant disadvantages.
Polyethylene terephthalate polymers can be either crystalline or non-crystalline solids. Crystalline polyethylene terephthalate (PET) is white and opaque, while the non-crystalline form is generally clear or transparent. When manufacturing a film the melted PET polymer may be extruded onto a casting roll, and if a molded article is to be manufactured, the PET polymer may be injected into a mold. Regardless of whether a film or molded article is being manufactured, the manufacturer desires to obtain the finished article substantially composed of transparent or clear PET to avoid the commercial disadvantage associated with a white, opaque film or article. It is known that when PET is heated to a semimolten state, above its crystalline melting point, PET is amorphous. Unless the polymer is quickly cooled, the film or molded article will tend to crystallize and the resulting product will have an opacity which is commercially unacceptable. The manufacturer is well aware of the fact that, when processing PET, he has only a limited amount of time, generally only about one minute, to solidify the film or molded article in the amorphous state before significant crystallization sets in.